Ras palmitoylation is necessary for N-Ras activation and signal propagation in growth factor signalling

Deacylases-structure, function, and relationship to diseases

Reversible S-acylation plays a pivotal role in various biological processes, modulating protein functions such as subcellular localization, protein stability/activity, and protein-protein interactions. These modifications are mediated by acyltransferases and deacylases, among which the most abundant modification is S-palmitoylation. Growing evidence has shown that this rivalrous pair of modifications, occurring in a reversible cycle, is essential for various biological functions. Aberrations in this process have been associated with various diseases, including cancer, neurological disorders, and immune diseases. This underscores the importance of studying enzymes involved in acylation and deacylation to gain further insights into disease pathogenesis and provide novel strategies for disease treatment. In this Review, we summarize our current understanding of the structure and physiological function of deacylases, highlighting their pivotal roles in pathology. Our aim is to provide insights for further clinical applications.

RASless MEFs as a Tool to Study RAS-Dependent and RAS-Independent Functions

RAS proteins are key players in multiple cellular processes. To study the role of RAS proteins individually or in combination, we have developed MEFs that can be rendered RASless, i.e., devoid of all endogenous RAS isoforms. These cells have significantly contributed to our understanding of the requirements for RAS functions in cell proliferation as well as their implications in diverse cellular processes. Here, we describe methods using RASless MEFs to study RAS-dependent cellular activities with special emphasis on proliferation. We provide the details to identify inducers of RAS-independent proliferation in colony assays. We recommend following these stringent guidelines to avoid false-positive results. Moreover, this protocol can be adapted to generate RASless MEFs ectopically expressing RAS variants to interrogate their function in the absence of endogenous RAS isoforms or to perform experiments in the absence of RAS. Finally, we also describe protocols to generate and use RASless MEFs for cell cycle analyses using the FUCCI cell cycle indicator.

Protein Palmitoylation in Leukocyte Signaling and Function

Palmitoylation is a post-translational modification (PTM) based on thioester-linkage between palmitic acid and the cysteine residue of a protein. This covalent attachment of palmitate is reversibly and dynamically regulated by two opposing sets of enzymes: palmitoyl acyltransferases containing a zinc finger aspartate-histidine-histidine-cysteine motif (PAT-DHHCs) and thioesterases. The reversible nature of palmitoylation enables fine-tuned regulation of protein conformation, stability, and ability to interact with other proteins. More importantly, the proper function of many surface receptors and signaling proteins requires palmitoylation-meditated partitioning into lipid rafts. A growing number of leukocyte proteins have been reported to undergo palmitoylation, including cytokine/chemokine receptors, adhesion molecules, pattern recognition receptors, scavenger receptors, T cell co-receptors, transmembrane adaptor proteins, and signaling effectors including the Src family of protein kinases. This review provides the latest findings of palmitoylated proteins in leukocytes and focuses on the functional impact of palmitoylation in leukocyte function related to adhesion, transmigration, chemotaxis, phagocytosis, pathogen recognition, signaling activation, cytotoxicity, and cytokine production.

The X-Linked Intellectual Disability Gene Zdhhc9 Is Essential for Dendrite Outgrowth and Inhibitory Synapse Formation

Palmitoylation is a reversible post-translational lipid modification that facilitates vesicular transport and subcellular localization of modified proteins. This process is catalyzed by ZDHHC enzymes that are implicated in several neurological and neurodevelopmental disorders. Loss-of-function mutations in ZDHHC9 have been identified in patients with X-linked intellectual disability (XLID) and associated with increased epilepsy risk. Loss of Zdhhc9 function in hippocampal cultures leads to shorter dendritic arbors and fewer inhibitory synapses, altering the ratio of excitatory-to-inhibitory inputs formed onto Zdhhc9-deficient cells. While Zdhhc9 promotes dendrite outgrowth through the palmitoylation of the GTPase Ras, it promotes inhibitory synapse formation through the palmitoylation of another GTPase, TC10. Zdhhc9 knockout mice exhibit seizure-like activity together with increased frequency and amplitude of both spontaneous and miniature excitatory and inhibitory postsynaptic currents. These findings present a plausible mechanism for how the loss of ZDHHC9 function may contribute to XLID and epilepsy.

Inhibition of NRAS Signaling in Melanoma through Direct Depalmitoylation Using Amphiphilic Nucleophiles

Activating mutations in the small GTPase NRAS are responsible for driving tumor growth in several cancers. Unfortunately, the development of NRAS inhibitors has proven difficult due to the lack of hydrophobic binding pockets on the protein's surface. To overcome this limitation, we chose to target the post-translational S-palmitoyl modification of NRAS, which is required for its signaling activity. Utilizing an amphiphile-mediated depalmitoylation (AMD) strategy, we demonstrate the ability to directly cleave S-palmitoyl groups from NRAS and inhibit its function. C8 alkyl cysteine causes a dose-dependent decrease in NRAS palmitoylation and inhibits downstream signaling in melanoma cells with an activating mutation in NRAS. This compound reduces cell growth in NRAS-driven versus non-NRAS-driven melanoma lines and inhibits tumor progression in an NRAS-mutated melanoma xenograft mouse model. Our work demonstrates that AMD can effectively suppress NRAS activity and could represent a promising new avenue for discovering lead compounds for treatment of NRAS-driven cancers.

ZDHHC12-mediated claudin-3 S-palmitoylation determines ovarian cancer progression

The membrane protein claudin-3 (CLDN3) is critical for the formation and maintenance of tight junction and its high expression has been implicated in dictating malignant progression in various cancers. However, the post-translational modification of CLDN3 and its biological function remains poorly understood. Here, we report that CLDN3 is positively correlated with ovarian cancer progression both in vitro and in vivo. Of interest, CLDN3 undergoes S-palmitoylation on three juxtamembrane cysteine residues, which contribute to the accurate plasma membrane localization and protein stability of CLDN3. Moreover, the deprivation of S-palmitoylation in CLDN3 significantly abolishes its tumorigenic promotion effect in ovarian cancer cells. By utilizing the co-immunoprecipitation assay, we further identify ZDHHC12 as a CLDN3-targating palmitoyltransferase from 23 ZDHHC family proteins. Furthermore, the knockdown of ZDHHC12 also significantly inhibits CLDN3 accurate membrane localization, protein stability and ovarian cancer cells tumorigenesis. Thus, our work reveals S-palmitoylation as a novel regulatory mechanism that modulates CLDN3 function, which implies that targeting ZDHHC12-mediated CLDN3 S-palmitoylation might be a potential strategy for ovarian cancer therapy.

Therapeutic targeting of protein S-acylation for the treatment of disease

The post-translational modification protein S-acylation (commonly known as palmitoylation) plays a critical role in regulating a wide range of biological processes including cell growth, cardiac contractility, synaptic plasticity, endocytosis, vesicle trafficking, membrane transport and biased-receptor signalling. As a consequence, zDHHC-protein acyl transferases (zDHHC-PATs), enzymes that catalyse the addition of fatty acid groups to specific cysteine residues on target proteins, and acyl proteins thioesterases, proteins that hydrolyse thioester linkages, are important pharmaceutical targets. At present, no therapeutic drugs have been developed that act by changing the palmitoylation status of specific target proteins. Here, we consider the role that palmitoylation plays in the development of diseases such as cancer and detail possible strategies for selectively manipulating the palmitoylation status of specific target proteins, a necessary first step towards developing clinically useful molecules for the treatment of disease.

Ras signals principally via Erk in G1 but cooperates with PI3K/Akt for Cyclin D induction and S-phase entry

Numerous studies exploring oncogenic Ras or manipulating physiological Ras signalling have established an irrefutable role for Ras as driver of cell cycle progression. Despite this wealth of information the precise signalling timeline and effectors engaged by Ras, particularly during G1, remain obscure as approaches for Ras inhibition are slow-acting and ill-suited for charting discrete Ras signalling episodes along the cell cycle. We have developed an approach based on the inducible recruitment of a Ras-GAP that enforces endogenous Ras inhibition within minutes. Applying this strategy to inhibit Ras stepwise in synchronous cell populations revealed that Ras signaling was required well into G1 for Cyclin D induction, pocket protein phosphorylation and S-phase entry, irrespective of whether cells emerged from quiescence or G2/M. Unexpectedly, Erk, and not PI3K/Akt or Ral was activated by Ras at mid-G1, albeit PI3K/Akt signalling was a necessary companion of Ras/Erk for sustaining cyclin-D levels and G1/S transition. Our findings chart mitogenic signaling by endogenous Ras during G1 and identify limited effector engagement restricted to Raf/MEK/Erk as a cogent distinction from oncogenic Ras signalling.

RAS at the Golgi antagonizes malignant transformation through PTPRκ-mediated inhibition of ERK activation

RAS GTPases are frequently mutated in human cancer. H- and NRAS isoforms are distributed over both plasma-membrane and endomembranes, including the Golgi complex, but how this organizational context contributes to cellular transformation is unknown. Here we show that RAS at the Golgi is selectively activated by apoptogenic stimuli and antagonizes cell survival by suppressing ERK activity through the induction of PTPRκ, which targets CRAF for dephosphorylation. Consistently, in contrast to what occurs at the plasma-membrane, RAS at the Golgi cannot induce melanoma in zebrafish. Inactivation of PTPRκ, which occurs frequently in human melanoma, often coincident with TP53 inactivation, accelerates RAS-ERK pathway-driven melanomagenesis in zebrafish. Likewise, tp53 disruption in zebrafish facilitates oncogenesis driven by RAS from the Golgi complex. Thus, RAS oncogenic potential is strictly dependent on its sublocalization, with Golgi complex-located RAS antagonizing tumor development.

RAS isoforms are associated with the plasma membrane and endomembranes, but how their localization contributes to tumorigenesis is unclear. Here, the authors show that RAS signals from Golgi complex antagonize tumour formation by inducing apoptosis via ERK inhibition.

Click-Chemistry Based High Throughput Screening Platform for Modulators of Ras Palmitoylation

Palmitoylation is a widespread, reversible lipid modification that has been implicated in regulating a variety of cellular processes. Approximately one thousand proteins are annotated as being palmitoylated, and for some of these, including several oncogenes of the Ras and Src families, palmitoylation is indispensable for protein function. Despite this wealth of disease-relevant targets, there are currently few effective pharmacological tools to interfere with protein palmitoylation. One reason for this lack of development is the dearth of assays to efficiently screen for small molecular inhibitors of palmitoylation. To address this shortcoming, we have developed a robust, high-throughput compatible, click chemistry-based approach to identify small molecules that interfere with the palmitoylation of Ras, a high value therapeutic target that is mutated in up to a third of human cancers. This assay design shows excellent performance in 384-well format and is sensitive to known, non-specific palmitoylation inhibitors. Further, we demonstrate an ideal counter-screening strategy, which relies on a target peptide from an unrelated protein, the Src-family kinase Fyn. The screening approach described here provides an integrated platform to identify specific modulators of palmitoylated proteins, demonstrated here for Ras and Fyn, but potentially applicable to pharmaceutical targets involved in a variety of human diseases.

FASN Inhibition and Taxane Treatment Combine to Enhance Anti-tumor Efficacy in Diverse Xenograft Tumor Models through Disruption of Tubulin Palmitoylation and Microtubule Organization and FASN Inhibition-Mediated Effects on Oncogenic Signaling and Gene Expression

Palmitate, the enzymatic product of FASN, and palmitate-derived lipids support cell metabolism, membrane architecture, protein localization, and intracellular signaling. Tubulins are among many proteins that are modified post-translationally by acylation with palmitate. We show that FASN inhibition with TVB-3166 or TVB-3664 significantly reduces tubulin palmitoylation and mRNA expression. Disrupted microtubule organization in tumor cells is an additional consequence of FASN inhibition. FASN inhibition combined with taxane treatment enhances inhibition of in vitro tumor cell growth compared to treatment with either agent alone. In lung, ovarian, prostate, and pancreatic tumor xenograft studies, FASN inhibition and paclitaxel or docetaxel combine to inhibit xenograft tumor growth with significantly enhanced anti-tumor activity. Tumor regression was observed in 3 of 6 tumor xenograft models. FASN inhibition does not affect cellular taxane concentration in vitro. Our data suggest a mechanism of enhanced anti-tumor activity of the FASN and taxane drug combination that includes inhibition of tubulin palmitoylation and disruption of microtubule organization in tumor cells, as well as a sensitization of tumor cells to FASN inhibition-mediated effects that include gene expression changes and inhibition of β-catenin. Together, the results strongly support investigation of combined FASN inhibition and taxane treatment as a therapy for a variety of human cancers.

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FASN inhibition decreases tubulin palmitoylation and disrupts microtubules in tumor cells but not non-tumor cells

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Combined FASN inhibition and taxane treatment increases inhibition of in vitro tumor cell colony growth

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FASN inhibition does not affect intracellular paclitaxel concentrations

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Combined FASN inhibition and taxane treatment significantly increases inhibition of tumor growth or causes regression of diverse xenograft tumors

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Taxane treatment sensitizes xenograft tumors to FASN inhibition-mediated beta-catenin blockade and gene expression changes

Fatty acid synthase (FASN) is a vital enzyme in tumor cell biology; the over-expression of FASN is associated with diminished patient prognosis and resistance to many cancer therapies. Our data demonstrate that selective and potent FASN inhibition combines with taxane treatment to enhance tumor growth inhibition and induce tumor regression in varied preclinical tumor models. Mechanism-of-action studies indicate that the increased activity of the combination results from the effects of both drugs. The results support clinical investigation of combined FASN inhibition and taxane treatment as an anti-cancer therapy.

How to Target Activated Ras Proteins: Direct Inhibition vs. Induced Mislocalization

Oncogenic Ras proteins are a driving force in a significant set of human cancers and wildtype, unmutated Ras proteins likely contribute to the malignant phenotype of many more. The overall challenge of targeting activated Ras proteins has great promise to treat cancer, but this goal has yet to be achieved. Significant efforts and resources have been committed to inhibiting Ras, but these energies have so far made little impact in the clinic. Direct attempts to target activated Ras proteins have faced many obstacles, including the fundamental nature of the gain-of-function oncogenic activity being produced by a loss-of-function at the biochemical level. Nevertheless, there has been very promising recent pre-clinical progress. The major strategy that has so far reached the clinic aimed to inhibit activated Ras indirectly through blocking its post-translational modification and inducing its mislocalization. While these efforts to indirectly target Ras through inhibition of farnesyl transferase (FTase) were rationally designed, this strategy suffered from insufficient attention to the distinctions between the isoforms of Ras. This led to subsequent failures in large-scale clinical trials targeting K-Ras driven lung, colon, and pancreatic cancers. Despite these setbacks, efforts to indirectly target activated Ras through inducing its mislocalization have persisted. It is plausible that FTase inhibitors may still have some utility in the clinic, perhaps in combination with statins or other agents. Alternative approaches for inducing mislocalization of Ras through disruption of its palmitoylation cycle or interaction with chaperone proteins are in early stages of development.

The residue at position 5 of the N-terminal region of Src and Fyn modulates their myristoylation, palmitoylation, and membrane interactions

Using biophysical methods in live cells and palmitoylation mutants of Src and Fyn, we show that palmitoylation stabilizes the interactions of SFKs with the plasma membrane. Moreover, we show that the amino acid at position 5 regulates the myristoylation and palmitoylation of these proteins, and thereby their targeting to raft domains.

The interactions of Src family kinases (SFKs) with the plasma membrane are crucial for their activity. They depend on their fatty-acylated N-termini, containing N-myristate and either a polybasic cluster (in Src) or palmitoylation sites (e.g., Fyn). To investigate the roles of these moieties in SFK membrane association, we used fluorescence recovery after photobleaching beam-size analysis to study the membrane interactions of c-Src-GFP (green fluorescent protein) or Fyn-GFP fatty-acylation mutants. Our studies showed for the first time that the membrane association of Fyn is more stable than that of Src, an effect lost in a Fyn mutant lacking the palmitoylation sites. Unexpectedly, Src-S3C/S6C (containing cysteines at positions 3/6, which are palmitoylated in Fyn) exhibited fast cytoplasmic diffusion insensitive to palmitoylation inhibitors, suggesting defective fatty acylation. Further replacement of the charged Lys-5 by neutral Gln to resemble Fyn (Src-S3C/S6C/K5Q) restored Fyn-like membrane interactions, indicating that Lys-5 in the context of Src-S3C/S6C interferes with its myristoylation/palmitoylation. This was validated by direct myristoylation and palmitoylation studies, which indicated that the residue at position 5 regulates the membrane interactions of Src versus Fyn. Moreover, the palmitoylation levels correlated with targeting to detergent-resistant membranes (rafts) and to caveolin-1. Palmitoylation-dependent preferential containment of Fyn in rafts may contribute to its lower transformation potential.

Defined spatiotemporal features of RAS-ERK signals dictate cell fate in MCF-7 mammary epithelial cells

Signals conveyed through the RAS-ERK pathway are essential for the determination of cell fate. It is well established that signal variability is achieved in the different microenvironments in which signals unfold. It is also known that signal duration is critical for decisions concerning cell commitment. However, it is unclear how RAS-ERK signals integrate time and space in order to elicit a given biological response. To investigate this, we used MCF-7 cells, in which EGF-induced transient ERK activation triggers proliferation, whereas sustained ERK activation in response to heregulin leads to adipocytic differentiation. We found that both proliferative and differentiating signals emanate exclusively from plasma membrane-disordered microdomains. Of interest, the EGF signal can be transformed into a differentiating stimulus by HRAS overexpression, which prolongs ERK activation, but only if HRAS localizes at disordered membrane. On the other hand, HRAS signals emanating from the Golgi complex induce apoptosis and can prevent heregulin-induced differentiation. Our results indicate that within the same cellular context, RAS can exert different, even antagonistic, effects, depending on its sublocalization. Thus cell destiny is defined by the ability of a stimulus to activate RAS at the appropriate sublocalization for an adequate period while avoiding switching on opposing RAS signals.

Targeting RAS Membrane Association: Back to the Future for Anti-RAS Drug Discovery?

RAS proteins require membrane association for their biologic activity, making this association a logical target for anti-RAS therapeutics. Lipid modification of RAS proteins by a farnesyl isoprenoid is an obligate step in that association, and is an enzymatic process. Accordingly, farnesyltransferase inhibitors (FTI) were developed as potential anti-RAS drugs. The lack of efficacy of FTIs as anticancer drugs was widely seen as indicating that blocking RAS membrane association was a flawed approach to cancer treatment. However, a deeper understanding of RAS modification and trafficking has revealed that this was an erroneous conclusion. In the presence of FTIs, KRAS and NRAS, which are the RAS isoforms most frequently mutated in cancer, become substrates for alternative modification, can still associate with membranes, and can still function. Thus, FTIs failed not because blocking RAS membrane association is an ineffective approach, but because FTIs failed to accomplish that task. Recent findings regarding RAS isoform trafficking and the regulation of RAS subcellular localization have rekindled interest in efforts to target these processes. In particular, improved understanding of the palmitoylation/depalmitoylation cycle that regulates RAS interaction with the plasma membrane, endomembranes, and cytosol, and of the potential importance of RAS chaperones, have led to new approaches. Efforts to validate and target other enzymatically regulated posttranslational modifications are also ongoing. In this review, we revisit lessons learned, describe the current state of the art, and highlight challenging but promising directions to achieve the goal of disrupting RAS membrane association and subcellular localization for anti-RAS drug development. Clin Cancer Res; 21(8); 1819-27. ©2015 AACR. See all articles in this CCR Focus section, "Targeting RAS-Driven Cancers."

ABHD17 proteins are novel protein depalmitoylases that regulate N-Ras palmitate turnover and subcellular localization

Dynamic changes in protein S-palmitoylation are critical for regulating protein localization and signaling. Only two enzymes - the acyl-protein thioesterases APT1 and APT2 – are known to catalyze palmitate removal from cytosolic cysteine residues. It is unclear if these enzymes act constitutively on all palmitoylated proteins, or if additional depalmitoylases exist. Using a dual pulse-chase strategy comparing palmitate and protein half-lives, we found knockdown or inhibition of APT1 and APT2 blocked depalmitoylation of Huntingtin, but did not affect palmitate turnover on postsynaptic density protein 95 (PSD95) or N-Ras. We used activity profiling to identify novel serine hydrolase targets of the APT1/2 inhibitor Palmostatin B, and discovered that a family of uncharacterized ABHD17 proteins can accelerate palmitate turnover on PSD95 and N-Ras. ABHD17 catalytic activity is required for N-Ras depalmitoylation and re-localization to internal cellular membranes. Our findings indicate that the family of depalmitoylation enzymes may be substantially broader than previously believed.

DOI:http://dx.doi.org/10.7554/eLife.11306.001

Proteins play important roles in many processes in cells. Some of these proteins can be modified by the addition of a molecule called palmitate. This process, termed “palmitoylation”, helps direct these proteins to the compartments within the cell where they are needed to carry out their roles. One target of palmitoylation is N-Ras, which is a protein that can promote the development of cancer.

We understand quite a lot about how palmitate is added to proteins, but much less about how it is removed. So far, researchers have only identified two enzymes – known as APT1 and APT2 – that can remove palmitate from proteins, but it is possible that there are others. Identifying other “depalmitoylase” enzymes could help us find ways to block the removal of palmitate from N-Ras, which could lead to new treatments for some cancers.

Lin and Conibear used several biochemical techniques to search for depalmitoylase enzymes in human cells. The experiments reveal that although APT1 and APT2 are important for removing palmitate from some proteins, they are not needed to remove palmitate from N-Ras. Instead, Lin and Conibear found that an enzyme called ABHD17 removes palmitate from N-Ras. The next step following on from this work will be to find out what other proteins ABHD17 acts on in cells. A longer-term challenge will be to develop specific chemicals that inhibit ABHD17 activity and test if they are able to reduce the growth of cancer cells.

DOI:http://dx.doi.org/10.7554/eLife.11306.002

Pharmacological Inhibition of Protein Lipidation

Lipid modifications of mammalian proteins are widespread, modifying thousands of targets involved in all aspects of cellular physiology cellular physiology. Broadly, lipidations serve to increase protein hydrophobicity and association with cellular membranes. Often, these modifications are absolutely essential for protein stability and localization, and serve critical roles in dynamic regulation of protein function. A number of lipidated proteins are associated with diseases, including parasite infections, neurological diseases, diabetes, and cancer, suggesting that lipid modifications represent potentially attractive targets for pharmacological intervention. This review briefly describes the various types of posttranslational protein lipid modifications, proteins modified by them, and the enzymatic machinery associated with these. We then discuss several case studies demonstrating successful development of lipidation inhibitors of potential (and more rarely, realized) clinical value. Although this field remains in its infancy, we believe these examples demonstrate the potential utility of targeting protein lipidation as a viable strategy for inhibiting the function of pathogenic proteins.

Structural determinants of protein partitioning into ordered membrane domains and lipid rafts

Increasing evidence supports the existence of lateral nanoscopic lipid domains in plasma membranes, known as lipid rafts. These domains preferentially recruit membrane proteins and lipids to facilitate their interactions and thereby regulate transmembrane signaling and cellular homeostasis. The functionality of raft domains is intrinsically dependent on their selectivity for specific membrane components; however, while the physicochemical determinants of raft association for lipids are known, very few systematic studies have focused on the structural aspects that guide raft partitioning of proteins. In this review, we describe biophysical and thermodynamic aspects of raft-mimetic liquid ordered phases, focusing on those most relevant for protein partitioning. Further, we detail the variety of experimental models used to study protein-raft interactions. Finally, we review the existing literature on mechanisms for raft targeting, including lipid post-translational modifications, lipid binding, and transmembrane domain features. We conclude that while protein palmitoylation is a clear raft-targeting signal, few other general structural determinants for raft partitioning have been revealed, suggesting that many discoveries lie ahead in this burgeoning field.

Inhibition of de novo Palmitate Synthesis by Fatty Acid Synthase Induces Apoptosis in Tumor Cells by Remodeling Cell Membranes, Inhibiting Signaling Pathways, and Reprogramming Gene Expression

Inhibition of de novo palmitate synthesis via fatty acid synthase (FASN) inhibition provides an unproven approach to cancer therapy with a strong biological rationale. FASN expression increases with tumor progression and associates with chemoresistance, tumor metastasis, and diminished patient survival in numerous tumor types. TVB-3166, an orally-available, reversible, potent, and selective FASN inhibitor induces apoptosis, inhibits anchorage-independent cell growth under lipid-rich conditions, and inhibits in-vivo xenograft tumor growth. Dose-dependent effects are observed between 20–200 nM TVB-3166, which agrees with the IC₅₀ in biochemical FASN and cellular palmitate synthesis assays. Mechanistic studies show that FASN inhibition disrupts lipid raft architecture, inhibits biological pathways such as lipid biosynthesis, PI3K–AKT–mTOR and β-catenin signal transduction, and inhibits expression of oncogenic effectors such as c-Myc; effects that are tumor-cell specific. Our results demonstrate that FASN inhibition has anti-tumor activities in biologically diverse preclinical tumor models and provide mechanistic and pharmacologic evidence that FASN inhibition presents a promising therapeutic strategy for treating a variety of cancers, including those expressing mutant K-Ras, ErbB2, c-Met, and PTEN. The reported findings inform ongoing studies to link mechanisms of action with defined tumor types and advance the discovery of biomarkers supporting development of FASN inhibitors as cancer therapeutics.

Research in context

Fatty acid synthase (FASN) is a vital enzyme in tumor cell biology; the over-expression of FASN is associated with diminished patient prognosis and resistance to many cancer therapies. Our data demonstrate that selective and potent FASN inhibition with TVB-3166 leads to selective death of tumor cells, without significant effect on normal cells, and inhibits in vivo xenograft tumor growth at well-tolerated doses. Candidate biomarkers for selecting tumors highly sensitive to FASN inhibition are identified. These preclinical data provide mechanistic and pharmacologic evidence that FASN inhibition presents a promising therapeutic strategy for treating a variety of cancers.

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TVB-3166 is an orally available, reversible, potent, and selective FASN inhibitor.

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TVB-3166 inhibition of FASN induces apoptosis in tumor cells but not normal cells.

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TVB-3166 inhibits in vivo xenograft tumor growth.

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Candidate biomarkers to select tumors sensitive to FASN inhibition are identified.

Systematic siRNA Screen Unmasks NSCLC Growth Dependence by Palmitoyltransferase DHHC5

Protein S-palmitoylation is a widespread and dynamic posttranslational modification that regulates protein-membrane interactions, protein-protein interactions, and protein stability. A large family of palmitoyl acyl transferases, termed the DHHC family due to the presence of a common catalytic motif, catalyzes S-palmitoylation; the role of these enzymes in cancer is largely unexplored. In this study, an RNAi-based screen targeting all 23 members of the DHHC family was conducted to examine the effects on the growth in non-small cell lung cancer (NSCLC). Interestingly, siRNAs directed against DHHC5 broadly inhibited the growth of multiple NSCLC lines but not normal human bronchial epithelial cell (HBEC) lines. Silencing of DHHC5 by lentivirus-mediated expression of DHHC5 shRNAs dramatically reduced in vitro cell proliferation, colony formation, and cell invasion in a subset of cell lines that were examined in further detail. The phenotypes were restored by transfection of a wild-type DHHC5 plasmid but not by a plasmid expressing a catalytically inactive DHHC5. Tumor xenograft formation was severely inhibited by DHHC5 knockdown and rescued by DHHC5 expression, using both a conventional and tetracycline-inducible shRNA. These data indicate that DHHC5 has oncogenic capacity and contributes to tumor formation in NSCLC, thus representing a potential novel therapeutic target.

IMPLICATIONS

Inhibitors of DHHC5 enzyme activity may inhibit non-small cell lung cancer growth.

Distinct palmitoylation events at the amino-terminal conserved cysteines of Env7 direct its stability, localization, and vacuolar fusion regulation in S. cerevisiae

Palmitoylation at cysteine residues is the only known reversible form of lipidation and has been implicated in protein membrane association as well as function. Many palmitoylated proteins have regulatory roles in dynamic cellular processes, including membrane fusion. Recently, we identified Env7 as a conserved and palmitoylated protein kinase involved in negative regulation of membrane fusion at the lysosomal vacuole. Env7 contains a palmitoylation consensus sequence, and substitution of its three consecutive cysteines (Cys(13)-Cys(15)) results in a non-palmitoylated and cytoplasmic Env7. In this study, we further dissect and define the role(s) of individual cysteines of the consensus sequence in various properties of Env7 in vivo. Our results indicate that more than one of the cysteines serve as palmitoylation substrates, and any pairwise combination is essential and sufficient for near wild type levels of Env7 palmitoylation, membrane localization, and phosphorylation. Furthermore, individually, each cysteine can serve as a minimum requirement for distinct aspects of Env7 behavior and function in cells. Cys(13) is sufficient for membrane association, Cys(15) is essential for the fusion regulatory function of membrane-bound Env7, and Cys(14) and Cys(15) are redundantly essential for protection of membrane-bound Env7 from proteasomal degradation. A role for Cys(14) and Cys(15) in correct sorting at the membrane is also discussed. Thus, palmitoylation at the N-terminal cysteines of Env7 directs not only its membrane association but also its stability, phosphorylation, and cellular function.